The present invention relates generally to devices for feeding reproducing transducers in apparatuses for reproducing rotary recording mediums. More particularly, the invention relates to a device which, in an apparatus for reproducing an information signal from a rotary recording medium by means of a reproducing transducer, moves the reproducing transducer in feeding travel along the radial direction of the rotary recording medium in a manner such that the transducer always carries out excellent tracking of the track on the recording medium.
Known system for recording and reproducing information signals (such as video signals and/or audio signals) on and from disc-shaped, rotary mediums (referred to hereinafter simply as discs) are broadly divided into systems using optical reproduction, systems using a reproducing stylus employing a piezoelectric element, and systems utilizing variations in the electrostatic capcitance between an electrode on the reproducing stylus and the recorded surface of the disc.
In accordance with a known electrostatic capacitance system, recording is accomplished by forming a spiral guide groove in the disc for guiding the reproducing stylus. As the same time, pits are formed in responsive correspondence with an information signal to be recorded on the bottom surface of the groove. In the reproducing system, the reproducing stylus is guided by the guide groove. The stylus traces the track within the groove and reproduces the signal in response to variations in the electrostatic capacitance.
In this known system, however, a reproducing stylus guiding groove is provided on the disc. The reproducing stylus is compulsorily guided by this guide groove. It is not possible for the reproducing stylus to undergo operations such as riding over the groove wall of one track, moving into another track and returning to the original track. If the reproducing stylus were to be forced to undergo such an operation, the groove and the stylus would be damaged. For this reason, it has not been possible in this known system to carry out special reproduction such as quick-motion picture reproductin, slow-motion picture reproduction, still-picture reproduction, and random access motion.
Accordingly, with the aim of overcoming the various problems of the known systems described above, a novel "Information signal recording and reproducing system" is disclosed in the specification of U.S. patent application Ser. No. 785,095, filed Apr. 6, 1977 and in the specification of U.S. patent application Ser. No. 33,324, filed Apr. 25, 1979, which is a continuation-in-part of the parent application Ser. No. 785,095. According to this system, the recording system forms pits in accordance with the information signal being recorded along a spiral track on a flat disc shaped recording medium, without forming a groove therein. In the reproducing system, a reproducing stylus traces over and along this track thereby to reproduce the recorded information signal in response to variations in the electrostatic capacitance.
By the use of this previous system, the recording track has no groove. There is no possibility whatsoever of the reproducing stylus or the recording medium being damaged. The stylus can trace the same portion of the track repeatedly many times, where, by a so-called random access motion, a reproducing stylus is moved at high speed to a desired position on a disc thereby to reproduce a desired information, in addition to a special reproduction such as still, slow motion, or quick motion reproduction.
In this system, since a groove for guiding the reproducing stylus is not provided on the disc, pilot or reference signals should be recorded on or in the vicinity of a track of the information signal, such as video signal, on a rotary disc. At the time of reproducing, the reference signals are reproduced together with the video signal. Tracking servo control is carried out so that the reproducing stylus traces accurately along the track in response to the reproduced reference signals.
In order to carry out a good tracking servo operation, it is desirable that the control operation be so accomplished that the output voltage of the control circuit is a voltage in the vicinity of the center of its control dynamic range. In general, the above mentioned reproducing stylus is mounted at the extreme free end of a cantilever, and, by causing the cantilever to undergo displacements, the tracking servo operation is so carried out that the reproducing stylus correctly traces the desired track. In this case, it is desirable that the tracking servo operation be carried out as the cantilever undergoes displacement with its neutral position as the center of motion.
However, in the press-moulding of a rotary recording medium (hereinafter referred to merely as a disc), in general, an offsetting (eccentricity) of its central hole occurs, although this may be slight. Furthermore, the central hole of a disc is formed to be of a diameter somewhat greater than the diameter of the upper spindle of the turntable on which the disc is placed. For these reasons, when the disc is placed on the turntable and rotated, the dimensional center of the disc and the center of rotation of the disc rotating together with the turntable do not coincide, strictly speaking, and are mutually eccentric in many cases.
On one hand, at the start of reproduction, the cantilever is in its neutral state when the reproducing stylus is lowered onto the disc. Then, when the disc is in an eccentric state as described above, tracking control is carried out in some extreme causes with the cantilever displaced to only one side from its neutral position depending on the timing of the lowering of the reproducing stylus as described hereinafter with reference to the drawings. In such a case, the control system, including the cantilever and the control circuit, must carry out a control operation by exceeding the limit of normal control operation, whereby there arises a problem in that normal tracking control operation cannot be accomplished.